According to rule 8.01, ‘pitchers shall take the sign from the catcher while standing on the rubber’. Unless there is a quick pitch situation, where they setup off then back on quickly to pitch, there doesn’t seem to be any penalty for the pitch signs coming from the dugout. Thus, the pitchers aren’t required to take the sign from the catcher.
We used to have fun with that, even before coaches decided pitchers should wear SuperSpy Secret Decoder Enigma Machines on their wrist.
“You’re pitcher isn’t taking signs from the catcher. The rule says she has to take signs from the catcher.”
“She certainly is taking signs from the catcher. Our signs are that if the catcher looks at you, she wants it waist-high down the middle. If she closes her eyes, hops up and down 3 times, and yells ‘Thunder-Chicken!’ she wants a 65 mph rise ball.”
But as you say, there’s no “only” after “catcher” so (perhaps sadly) there’s no prohibition on signs from the dugout. The intent of the rule is to institute a procedural pause to prevent a quick pitch and as long as the pitcher pauses — simulating taking a sign from the catcher, the sign does not need to be acknowledged in any way. At least that’s what I’ve heard in discussions over the years.